I think making Astarion's ark irredeemabale is a kind of slap for those who loves him but don't want to play as complete evil character. I mean, it is good to have a choice for both sides.
You don't always need to redeem someone to coexist peacefully. Even if your alignments are different. You still may find common ground in certain situations. And Larian promised more detailed personalities than just alignments.
